This invention relates to a network element, particularly to a network element for a synchronous digital communications network such as SDH or SONET.
Such a network element is known from an article by R. Castelli et al, “Technologie der SDH-Netzelemente: die Hardware-Plattform”, Elektrisches Nachrichtenwesen 4/1993, pp. 312-321. In FIGS. 9 and 10 of that article, a crossconnect for a digital communications network based on the Synchronous Digital Hierarchy (SDH) standard is shown. It has a number of interface modules and two switching matrices. Each of the switching matrices is connected to each of the interface modules and serves to switch paths between the interface modules. One of the switching matrices is always available as an active switching matrix, and the other is available as a standby matrix in the event of a failure. A controller detects a fault condition of the active switching matrix and switches to the standby matrix, which is then used as a new active switching matrix. Thus, the crossconnect uses 1:1 redundancy, which protects against hardware errors in the switching matrix, the core element of the crossconnect. For the interface modules, 1:N redundancy may additionally be provided, so that an individual standby module is available for a number N of interface modules to perform the function of a failed module.
It may happen, however, that as a result of fire, an explosion, or a flood disaster, all equipment in a room is destroyed and thus fails. The redundancy used in prior-art network elements cannot protect against such a disaster, because all modules of the network element are affected by the disaster at the same time.
To protect against such a disaster, it is known to install two like network elements in two separate rooms and connect them with one another. If one network element fails, at least part of the traffic, e.g., prioritized traffic such as emergency numbers, can still be handled by the second network element. Such a structure is shown in FIG. 1. Its disadvantages are the high capital expenditure for two complete pieces of equipment and the size of the switching matrices that are required to ensure that even with protection mechanisms such as MSP (Multiplexer Section Protection) and SNCP (Subnetwork Connection Protection), the network elements are 100% nonblocking.